1. Field of the Invention.
The present invention relates to amplifier circuits. In particular, the present invention is a circuit and method for input offset voltage trimming an input stage of an integrated circuit amplifier.
2. Description of the Prior Art.
In an integrated circuit operational amplifier, the input stage typically is a differential amplifier formed by a differential pair of transistors. Typically, the differential amplifier input stage exhibits an offset voltage of a random magnitude and polarity, which results in different output currents from the differential amplifier input stage when input voltages at the two input terminals are equal.
In order to make the output currents equal when the input voltages are equal, some form of input offset voltage trimming using adjustable resistors can be performed. Laser trimming of resistance networks on an integrated circuit chip is one approach which has been used in prior art input offset voltage trimming. The prior art techniques, however, typically require a large total resistance value (for example 500K-2000K ohms), which occupies a large portion of the chip area. These prior art trimming schemes, despite using large total resistor values, have typically covered only a small trimming range (about 0.5 mv-1 mv). In addition, laser trimming can affect the temperature coefficient of the offset voltage VOS, which results in poor long term stability of the amplifier circuit characteristics.